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Teardown: The inverter – from daylight to the ability grid – EDN.com

This article looks at the architectural design and components of a solar inverter card, starting with the DC inputs of the solar panel and going through the process of converting DC to AC to the AC output sent to the utility grid. We will show which functions need to be implemented in such a design in order to meet various safety and other performance standards, as well as strict requirements of energy providers for the signal that is fed into their network.

In doing so, we will select the most important elements and components that were taken in the development of the SMA series of solar inverters of the "Sunny Boy" series, from the EMI suppression capacitors from Vishay to the DSP TMS320F2812 from Texas Instruments, with a special focus on isolation and Protection through the intelligent use of optically isolated MOSFET gate drivers such as the HCPL-316J and HCPL-J312 from Avago.

Note: For further analysis on the frequently overlooked topic of optical isolation, we have included a detailed video interview (see below).

Photovoltaic (PV) systems consist of several components, such as B. PV solar modules that convert sunlight into electricity, mechanical and electrical connections and brackets and solar inverters, which are essential for the transmission of electricity generated by solar energy into the grid. illustration 1 shows a general, but comprehensive and complete block diagram of the photovoltaic system.


Figure 1: Complete Block Diagram of the Photovoltaic Plant (Courtesy Texas Instruments)

What is a PV solar inverter?

The main function of the inverter is to convert variable voltage direct current from sunlight on the PV modules or the battery storage into a specific alternating voltage and frequency for use by devices and return to the grid. The AC output will of course vary by region. In North America 115 Hz alternating current is used with 115 Hz and in large parts of Europe with 230 Hz alternating current with 50 Hz.

Enter SMA Solar Technology AG with headquarters in Germany and the solar inverter series "Sunny Boy". The inverter board that we are in Figure 2 is used in the transformerless versions Sunny Boy 3000TL, 4000TL and 5000TL, which are designed for AC output systems with 3 kW, 4 kW or 4.6 kW (at 230 V, 50 Hz).

The inverter card has multi-string technology with two independent DC / DC converters, making highly complex generator configurations easy to implement. This section of the input can be seen in Figure 2 in the lower left quadrant in the picture. Each of the two DC inputs uses Vishay EMI suppression capacitors # 339MKP as part of the filter, and the filter also includes DC common mode filter inductors wound on a common core, as well as a 15uF step-up smoothing capacitor # MKPC4AE that goes in The same figure below is shown to the left quadrant Figure 2 .

Two relays are also used on the DC input side to monitor the insulation resistance in accordance with IEC 61557-8 in pure IT AC systems. See Figure 2 upper left quadrant.

Insulation resistances between system lines and system earth are measured. If the adjustable threshold values ​​are not reached, the output relays switch to the error state.

With these relays, a superimposed DC measurement signal is used for measurement. The value of the insulation resistance of the system to be measured is calculated from the superimposed DC measurement voltage and the resulting current. Note the Hall effect current transducers in the diagram of Figure 2 .

One of the most impressive features of this SMA inverter card is the use of very high quality active and passive components that improve the reliability and performance of this inverter design.


Figure 2: Solar power inverter card from SMA Solar Technology AG for the "Sunny Boy" series.

Maximum credit point (MPP)

The first DC function in the signal chain is the MPP function.

This inverter task compensates for environmental conditions that affect the power output. For example, the output voltage and current of the PV module are very susceptible to fluctuations in temperature and light intensity per unit area (referred to as "irradiance"). The cell output voltage is inversely proportional to the cell temperature and the cell current is directly proportional to the irradiance.

The large variation in these and other key parameters causes the optimal operating point for the AC voltage and current of the inverter to move considerably. The inverter overcomes this problem by using a regulation to maintain operation at the so-called MPP, where the product of voltage and current has its highest value. SMA uses the OptiTrac Global Peak MPP tracker. The tried and tested operations tracker management system OptiTrac finds and uses the optimal operating point that delivers good yields with this additional function despite partial shading in PV systems. The TI DSP controller is the brain behind Maximum Power Point Tracking (MPPT).

The most common algorithm for determining the MPP is for the controller to perturb the operating voltage of the panel with each MPPT cycle and observe the output. The algorithm continues to oscillate over a sufficiently large area around the MPP to avoid local but misleading peaks in the power curve caused, for example, by movement in the cloud cover or some other condition affecting the curve. The disturbance and observation algorithm is inefficient in that it swings away from the MPP every cycle.

An alternative, the incremental inductance algorithm, solves the derivation of the power curve for 0, which by definition is a peak, and then settles on the resolved voltage level. While this approach does not have the inefficiency caused by oscillation, it risks other inefficiencies because it can settle on a local peak instead of the MPP. A combined approach maintains the level determined by the incremental inductance algorithm, but scans at intervals over a larger area to avoid selecting local peaks. While this approach is the most efficient, it also requires the greatest power from the controller.

Figure 3 shows how the determination of MPP can vary under different conditions.


Figure 3: MPP under various conditions such as weather, time of day, and plate heat (courtesy of Texas Instruments).

A capacitor is commonly used to store the energy that needs to be stored and accessed by the inverter. This capacitor is usually located on the PV bus and needs to be large enough to control the voltage ripple across the bus. Otherwise this ripple would have a detrimental effect on the MPPT accuracy.

Electrolytic capacitors are very suitable for controlling the ripple due to their low ESR (Equivalent Series Resistance) and the high capacitance per volume. Banks of smoothing capacitors can be seen in Figure 2 along the top of the circuit board.

Boost DC-DC boost converter

Next up is the DC / DC boost converter, which amplifies the DC input to the switching MOSFET bridge so that the inverter can efficiently generate a 230V 50Hz AC sine wave that can be sent to the grid. This DC / DC boost converter, together with the H5 switching bridge, is contained in the separate power module that is attached to the rear of the inverter card. This module is well cooled on the case. See the upper middle section of the board in Figure 2 where this module would be mounted in final assembly.

Figure 4 shows the essential basic DC / AC converter circuits or inverters in a typical transformerless configuration in which:

The DC / DC conversion increases or decreases the incoming PV voltage and adjusts its output to achieve the greatest efficiency in the DC / AC conversion stage
The capacitor provides further voltage buffering
The IGBTs or MOSFETs shown in the H4 bridge use a switching frequency in the range of 20 kHz to generate an AC voltage
The coils smooth the switched AC current into a sinusoidal signal to produce a line frequency AC output.

Transformerless inverter technology

The idea behind transformerless switching existed long before the development of the PV market. Device engineers know that a pair of field effect transistors operate most efficiently when fully ON or OFF when there is no current flowing through them and they are not consuming any current. Thus, amplifying an ideal square wave would theoretically be 100% efficient.

When a signal is modulated by a much higher frequency square wave, the result is pulse width modulation (PWM), and the corresponding circuit is called class D. In this way, it is possible to convert direct current to direct current or to switch direct current to AC efficiently. For solar inverters, the technology was not available in the past due to the high cost of the switching MOSFETs and IGBTs. However, these are getting cheaper and faster every year, so the technology has become more cost-effective than analog switching into large masses of copper and iron. The same technology makes electric cars possible.

Transformerless inverters have been available in Europe for several years and SMA received UL certification for sales in the USA in August 2010. The certification applies to the transformerless inverters Sunny Boy 8000TL-US, Sunny Boy 9000TL-US and Sunny Boy 10000TL-US from SMA and was granted on the basis of compliance with the "UL Standard 1741 for PV and battery-operated inverters". For the first time, this includes requirements for transformerless inverters. Transformerless inverters are considerably lighter than their galvanically isolated counterparts and, thanks to their advanced switching circuit, can offer a wider range of operating voltages than conventional inverters.


Figure 4: Transformerless DC / AC conversion circuit – the inverter (courtesy of Texas Instruments).


The disadvantage of non-galvanic isolation is the possibility that a ground fault could destroy the inverter and cause an electrical fire. If the secondary of a transformer is short-circuited, all of the current flows through the primary winding and is (hopefully) stopped by a thermal break as soon as the transformer overheats. Without one, if there is no protection, or if the protection fails to detect the ground fault and trip, the large MOSFETs or IGBTs will instantly fail in a rather catastrophic manner. Fortunately, the likelihood of such an event occurring is extremely small and all of these inverters must have earth fault protection as per the requirements of UL 1741. However, it remains the burden of the installer to ensure that in the event of an undetected earth fault, the regenerative current is taken into account when dimensioning the combiner and when disconnecting the fuses.

Provided the correct simple calculations are performed, transformerless inverters have few disadvantages and numerous advantages.

However, the PV inverter offers many other important functions.

The PV inverter also offers a grid disconnection function to prevent the PV system from supplying power to a power grid that has been disconnected. This means that an inverter that remains online via an unreliable connection during the grid shutdown or the power supply can lead to the PV system feeding back local grid transformers, generating thousands of volts on the power pole and putting utility workers at risk. The IEEE 1547 and UL 1741 safety standards stipulate that all grid-connected inverters must disconnect if the grid voltage or frequency is not within the specified limits, or must be switched off if the grid is no longer available. When reconnected, the inverter can only supply power if the inverter has determined the nominal voltage and frequency over a period of five minutes. This can be seen in the upper right quadrant of Figure 2 Use of four LF-G mains safety shut-off relays with 22 A, 250 VAC.

However, this is not the end of the inverter's duties. In addition to these tasks, the inverter also supports manual and automatic input / output separation for service companies, EMI / RFI suppression, ground fault interruption, PC-compatible communication interfaces (Bluetooth in this "Sunny Boy" series) and more. The inverter is housed in a robust housing and is expected to remain outdoors at full power for more than 25 years!

A typical single phase PV inverter like the SMA card uses a digital power regulator, the DSP, and a pair of high-side / low-side gate drivers to drive a full-bridge pulse width modulated (PWM) converter. In this and many good inverter applications, a full H-bridge topology is used because it has the highest load-bearing capacity of any switch mode topology. SMA uses H5 technology, in which a fifth power semiconductor between the input capacitor and the H-bridge prevents a loss that induces an oscillation of the electrical charge, and again significantly reduces the power loss. The H5 is a significant improvement over the classic inverter bridge circuit (H4 topology) and has maximum conversion efficiencies of 98%. In order to avoid a fluctuating potential of the PV generator, the architecture separates the DC side from the AC side during the free-running periods of the inverter.

The H5 topology shown in Figure 5 only needs one more switch compared to the normal full H4 bridge in Figure 4 . The switches T1, T2 and T4 are operated with a high frequency of around 20 kHz, T1 and T3 with a mains frequency, in this case 50 Hz. During freewheeling, T5 is open and separates the DC and AC sides. The freewheeling path is closed via T1 and the inverse diode of T3 for positive and T3 and the diode of T1 for negative current.


Figure 5: H5 bridge topology from SMA.


The PWM voltage switching action synthesizes a discrete but noisy 50 Hz current waveform at the full bridge output. The high-frequency noise components are inductively filtered and generate the sine wave with a moderately low amplitude and 50 Hz. The H-bridge works through asymmetrical unipolar modulation. The high side of the asymmetrical H-bridge should be driven by a 50 Hz half-wave, depending on the polarity of the network, while the opposite, low side is PWM-modulated to form the sinusoidal network shape. You will see the section AC output filters in Figure 2 with EMI suppression capacitor on the right side of the inverter board. The output sine filter with large inductances is also screwed to this card in this area to complete the AC filter.

The design of PV inverters requires many design compromises which, if wrongly compromised, can lead to heartburn in designers. For example, PV systems are expected to operate reliably and at full capacity for at least 25 years, and yet they must be offered at competitive prices, forcing the designer to make strict trade-offs between cost and reliability. PV systems require highly efficient inverters because higher-efficiency inverters run cooler and last longer than their less efficient counterparts, and they save money for both the manufacturer and the user of PV systems. SMA has done an exceptional job here.

The control architecture

The “brain” behind the inverter is its controller, in this case usually a digital power controller (DPC) or a digital signal processor (DSP). Controllers based on digital signal processors (DSP) such as the Texas Instruments TMS320F2812 in this design offer the high computing power and programming flexibility that are required for real-time signal processing in solar inverters. Highly integrated digital signal regulators help inverter manufacturers develop more efficient, lower-cost products that can support the growing demand for solar energy in the years to come.

A control processor for an inverter must overcome a number of real-time processing challenges in order to effectively execute the precise algorithms required for efficient DC / AC conversion and circuit protection. The MPPT and battery charge control require only a real-time reaction, but contain algorithms with a high degree of processing. Digital signal controllers that combine high performance DSPs and integrated control peripherals provide an excellent solution for real-time control of the DC / AC converter bridge, MPPT and protection circuit in solar inverters. DSP controllers inherently support high-speed mathematical calculations for use in real-time control algorithms.

Integrated peripheral devices such as analog-to-digital converters (ADCs) and pulse-width modulated outputs (PWMs) enable the direct acquisition of inputs and the control of power IGBTs or MOSFETs, which saves space and costs for the system. On-chip flash memory supports programming and data acquisition, and communication ports simplify the design for networking with devices such as meters and other inverters. The higher efficiency of DSP controllers in solar inverters has already been demonstrated by designs in which it has been reported that the losses in conversion efficiency have been reduced by more than 50 percent and a significant cost reduction has been achieved.

Typically, the controller firmware is implemented in a state machine format to ensure the most efficient execution with non-blocking (fall-through) code. This prevents the execution from accidentally getting into an infinite loop. The firmware execution is hierarchical and usually serves functions with the highest priority more often than functions with lower order. In the case of a PV inverter, the compensation of the isolated feedback loop and the modulation of the circuit breaker usually have the highest priority, followed by critical protection functions in support of safety standards and finally by the efficiency control or the Maximum Power Point (MPP). The remaining firmware tasks mainly concern the optimization of the operation at the current operating point, the monitoring of the system operation and the support of the system communication.

In addition to system operation, integrated functions ensure cost efficiency. TI's TMS320F2812 controller features ultra-fast 12-bit ADCs that provide up to 16 input channels for the current and voltage sensing necessary to achieve a regular sinusoidal waveform. For safety reasons, the ADCs can also provide current detection in the residual current device (RCD).

Twelve individually controlled Enhanced PWM (EPWM) channels offer variable duty cycles for high-speed switching in the converter bridge and the battery charging circuits. Each of the EPWMs has its own timer and phase register so that the phase delay can be programmed, and all EPWMs can be synchronized to drive multiple stages at the same frequency. Multiple timers allow access to multiple frequencies, and fast interrupt management is available to support additional control tasks. Several standard communication ports, including the CAN bus, provide easy interfaces to other components and systems.

isolation


Figure 6: Alternative energy systems require isolated connections (red) between the high-voltage circuits and the controller that manages the flow of electricity (courtesy of Avago).


Right in the middle of the SMA inverter card we find five isolated Avago gate drivers. See Figure 2 .

Two of the isolated MOSFET drivers that control the switching of T1 and T3 at a line frequency of 50 Hz are the Avago HCPL-316J, a 2.5 A gate drive optocoupler with integrated (VCE) desaturation detection and fault status feedback. The other three isolated MOSFET drivers that control the higher frequency switching of T2, T4 and T5 are Avago HCPL-J312, 2.5A output current MOSFET gate driver optocouplers. See Figure 5 for the H5 configuration.

Optocouplers offer reinforced insulation and fail-safe protection in the event of a fault, particularly with a transformerless inverter concept.

Why is reactive power control important in a PV inverter1?

The "Sunny Boy" models 3000TL / 4000TL / 5000TL are available with reactive power control.

Reactive power usually occurs whenever energy is transmitted via alternating current. Their importance for solar engineers and PV system operators is increasing, for both larger and smaller systems. Most important finding: reactive power is not a problem at all. It is actually a solution to some problems.

On July 1, 2010, PV systems in Germany that are fed into the grid at medium voltage level had to be able to feed reactive power into the grid. This is specified in the 2008 edition of the medium-voltage guidelines of the Federal Association of Energy and Water Management. Even stricter requirements are being discussed for the low-voltage network.

How is reactive power developing?

For direct current, the equation is quite simple: electrical energy is the product of voltage and current. With alternating current, however, things are a little more complicated because the intensity and direction of a current and a voltage change regularly here. See Figure 7 .


Figure 7: The required reactive power is generated in the inverter – in addition to the received PV real power. The geometric sum of both is the apparent force; it is critical to the inverter design. (Courtesy of SMA)


In the public grid, both have a sinusoidal trajectory with a frequency of 50 or 60 Hz. As long as the current and voltage are "in phase", i. H. Moving in the same rhythm, the product of these two oscillation factors is also an oscillation output with a positive average value – pure real power (Figure 8a ).


Figure 8a: If there is no phase shift, the product of current i and voltage u is an oscillating, but always positive output – pure active power. (Courtesy of SMA)


However, as soon as the sinusoidal trajectories of the current and the voltage are shifted from one another, their product will be an output with an alternating positive and negative sign. In extreme cases, the current and voltage are phase-shifted by a quarter period: the current always reaches its maximum intensity when the voltage is zero – and vice versa. The result: pure reactive power, positive and negative signs neutralize each other completely (Figure 8b ).


Figure 8b: A phase shift of 90 degrees between the current i and the voltage u results in an alternating positive and negative output with an average value of zero – pure reactive power. (Courtesy of SMA)


This phase shift can of course occur in two directions. It occurs when there are coils and capacitors in the AC circuit – which is usually the case: all motors or transformers have coils (for inductive switching); Capacitors (for capacitive displacement) are also often found.

Multi-conductor cables also work like a capacitor, while high-voltage overhead lines can be viewed as extremely long coils. Therefore, some degree of phase shift, i.e. H. Reactive power, cannot be avoided in alternating current networks. The measurement parameter for the phase shift is the shift factor cos (φ), which can have a value between 0 and 1. It can be used to easily convert output values. Reactive power is measured in a unit called Volt-Ampere-Reactive (VAR) and not in watts (see Formula 1).


Formula 1: reactive power calculation according to the Pythagorean theorem for right triangles. (Courtesy of SMA)


What effects does reactive power have on the grid?

Only real power is actually usable power. It can be used to power machines, light up lamps, or run electrical heaters. Reactive power is different: it cannot be consumed and therefore cannot supply electrical devices with electricity. It just moves back and forth in the grid and thus acts as an additional load. All cables, switches, transformers and other parts must also take reactive power into account.

This means that they must be designed for apparent power, the geometric sum of active and reactive power. The ohmic losses during power conduction occur based on the apparent power; additional reactive power therefore leads to greater line losses.

Video Interview: Jamshed Khan, Avago Optical Isolation Product Applications Engineer, discusses the importance of isolation in solar converters with EDN Editor Steve Taranovich.


Go forward

PV systems are relatively new in the field of power generation. Like other emerging technologies, PV systems will change rapidly as technology increases. As a result, PV systems will no doubt evolve to meet market demands for higher capacity, lower cost, and higher reliability. In this case, the capabilities of PV inverters will expand and developers will demand more integrated, application-specific devices at the component level. As these events progress, PV power systems will continue to spread and ultimately become a viable segment of the utility grid that will significantly reduce our dependence on fossil fuels.

References

From the SMA Solar Technology website: http://www.sma.de/en/products/knowledge-base/sma-shifts-the-phase.html
Texas Instruments Application Report No. SLVA446 – November 2010, "Introduction to Photovoltaic Systems Maximum Power Point Tracking"
Texas Instruments Application Report No. SPRAAE3 – May 2006, "DSP Controller TMS320C2000 ™: Perfect for Solar Inverters"
White Paper "Integrate Protection with Isolation into Renewable Energy Systems at Home"
“Analysis and modeling of transformerless photovoltaic inverter systems” by Tamás Kerekes, Institute for Energy Technology at Aalborg University, Denmark, August 2009


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